Governments and local administrations increasingly use the internet to improve citizens’ participation in deliberation processes. However, research studies have pointed out that deliberation outcomes vary due to the participants’ sociodemographic differences. In this paper, we address this debate by quantitatively measuring different sociodemographic participant groups’ deliberation quality. By building an index of the quality of understanding (IQU), we analyze the quality of 1,991 postings on local political issues that participants contributed during the 2011 Zurich City debate. We defined five indicators for deliberation quality: statement of reasons, proposals for solutions, respect, doubts, and reciprocity. The analysis confirms that the sociodemographic composition of the participants is of great importance for self-selected participation and deliberation quantity, but not for the deliberation quality.